Reduction in weight of a vehicle body has been advanced by using a high-strength steel sheet to suppress emission of carbon dioxide gas from a vehicle. Besides, a lot of high-strength steel sheets with a maximum tensile strength of 980 MPa or more have been used for the vehicle body in addition to a mild steel sheet to secure safety of passengers. Further, it is necessary to enhance a usage strength level of the high-strength steel sheet more than before to further advance the reduction in weight of the vehicle body. The high-strengthening of the steel sheet is generally accompanied by deterioration of material properties such as the low-temperature toughness, and therefore, it is important in development of the high-strength steel sheet how to enable the high-strengthening without deteriorating the material properties.
The fatigue strength of the steel sheet is a property required for important safety-related parts centering on steel sheets for underbody parts of the vehicle. The reduction in weight of parts cannot be enabled if the fatigue strength is not improved together with the high-strengthening of the steel sheet. Accordingly, fatigue properties are defined not only by the strength but also by a fatigue limit in which fatigue fracture does not occur due to a stress smaller than that, and a fatigue strength at finite life being a fatigue life when a certain stress or strain is loaded.
Besides, there is a requirement for the steel sheet used for the member as stated above that the member is difficult to be fractured even if it is impacted by a collision or the like after the steel sheet is formed as the member and attached to the vehicle. Further, there also is a requirement to improve the low-temperature toughness to secure impact resistance at a cold district. This low-temperature toughness is defined by vTrs (Charpy fracture appearance transition temperature) or the like. Therefore, it is necessary to consider the impact resistance in itself of the steel sheet. In addition, plastic deformation of the steel sheet becomes difficult due to the high-strengthening of the steel sheet, and therefore, anxiety for fracture becomes higher. Accordingly, toughness is required as an important property.
It is effective to refine a structure to improve the fatigue properties. For example, in Patent Literature 1 and Patent Literature 2, a hot-rolled steel sheet in which an average grain diameter of ferrite is set to be 2 μm or less, and a strength-ductility balance and a fatigue limit ratio (fatigue strength/TS) are good is disclosed. However, these steel sheets have a structure whose main phase is the ferrite, and it is difficult to secure the strength of 980 MPa or more.
Besides, fatigue cracks occur from a vicinity of a surface, and therefore, it is particularly important to refine the structure in the vicinity of the surface. For example, in Patent Literature 3, a hot-rolled steel sheet in which a main phase thereof is polygonal ferrite, an average crystal grain diameter of the polygonal ferrite becomes gradually small from a center of a sheet thickness toward a surface layer to be a crystal grain diameter inclined structure is disclosed. This hot-rolled steel sheet is one in which a polygonal ferrite fraction is gradually refined from the center part of the sheet thickness toward the surface layer part of the sheet thickness by applying bending after hot-rolling.
Further, in Patent Literature 4, a hot-rolled steel sheet is disclosed in which polygonal ferrite is a main phase, and a crystal grain diameter in a vicinity of a surface layer is set to be 20% or less of a grain diameter at a center part of a sheet thickness. This hot-rolled steel sheet is one in which rolling is performed in a ferrite region, the surface layer is reverse transformed by a process heat generation at the hot-rolling time to thereby refine the structure at the surface layer.
These hot-rolled steel sheets are excellent in the fatigue properties, but a manufacturing method thereof is complicated, preferred manufacturing conditions are narrow, and therefore, lowerings in productivity and yield are worried. Besides, the main phase is the ferrite, and therefore, it is difficult to secure the strength of 980 MPa or more.
On the other hand, a martensite structure is extremely hard, and therefore, in a steel sheet having high-strength of 980 MPa class or more, it is often the case that the martensite structure is used as the main phase or a second phase for strengthening. In Patent Literature 5, improvement in the fatigue properties by grain refining in a structure in which the martensite structure is the main phase is described. Note that this is an art in a steel-pipe field, and is one in which a diameter is reduced after a pipe-making, and an average block diameter of martensite is set to be 3 μm or less by quenching after heating. It is necessary to install a heating and quenching device after a finish rolling to apply the similar method for the hot-rolled steel sheet to improve the fatigue strength, and there is a problem in which a large investment is necessary.
On the other hand, for example, in Patent Literature 6, a manufacturing method is disclosed in which a martensite phase whose aspect ratio is adjusted is set to be a main phase of a structure of a steel sheet, as a method to improve the toughness of the steel sheet.
In general, it is known that the aspect ratio of the martensite depends on an aspect ratio of an austenite grain before transformation. Here, the martensite whose aspect ratio is large means the martensite transformed from non-recrystallized austenite (austenite which is extended by rolling). Besides, the martensite whose aspect ratio is small means the martensite transformed from recrystallized austenite.
Therefore, it is necessary for the steel sheet according to Patent Literature 6 to recrystallize the austenite to reduce the aspect ratio. In addition, it is necessary to increase a finish rolling temperature to enable the recrystallization of the austenite. Accordingly, there is a tendency in which a grain diameter of the austenite, furthermore a grain diameter of the martensite become large. In general, it is known that grain refining is effective to improve the toughness. Therefore, when the aspect ratio is lowered, it is possible to reduce a factor of toughness deterioration resulting from a shape, but it is accompanied by the toughness deterioration resulting from coarsening of a crystal grain diameter, and therefore, there is a limit in improvement in toughness. In addition, it is not mentioned as for the fatigue strength, and it is difficult to say that enough fatigue strength is secured.